1. Field of the Invention
This invention concerns vibrating tongs with an adaptable clamping force for mechanical fruit harvesting.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Mechanical harvesting of fruit and especially small fruits, such as, for example, olives, cherries, almonds, nuts, etc. using vibrating machines is now widely applied by a large number of fruit tree growers. Such machines include in particular a vibration head consisting, on the one hand, of a rugged pair of tongs designed to clamp the trunk and branches of fruit trees and consisting of two jaws or grips of which at least one is a moving jaw and the closing (clamping) and unclamping (opening) movements are achieved through one or more hydraulic jacks and, on the other hand, of a vibrator communicating vibrations to the jaws and allowing for the fruits to be detached. A vibration head of this kind is, for example, described in French Patent No. 2799610.
The resistance of the tree bark to the pressure of the tong jaws may vary considerably, especially in terms of the fruit species, varieties of one same species, the harvesting season, weather conditions, soil moisture (for example, depending on watering), etc.
The clamping force applied by the jaws raises a problem, because it can exert either to much pressure and the bark breaks, or it is too weak; and a substantial amount of effectiveness is lost.
Depending on species and varieties, there are harvesting periods that coincide with those times when the tree sap is rising, in other words, when the sap rises between the bark and the central cylinder of the tree trunk.
During these periods, the tree is more vulnerable because if its bark is damaged, the rise of the sap does not occur normally and the tree will wither quickly and irremediably.
In order to limit the magnitude of damage caused to the bark of trees, it has been proposed (in U.S. Pat. No. 3,555,799; 3,504,486; and 3,570,230) to limit the pressure to a predetermined value (for example 50 bars) in the oil intake chamber of the clamping jack or jacks. However, during activation periods of the vibrator, the pressure in this chamber is considerably increased by the push coming from the opposite direction exerted on the piston moving in said chamber. This results in:                an instability in the behavior of the clamping jack due to micro-displacements of the piston and the rod of this jack at a high frequency;        a seizing of the jack shaft in its guide bearing resulting from these high-frequency micro-displacements;        when the fasteners of the clamping jack are articulated, there is a harmful play in these joints; and        in a case when there was no flap valve, the pressure of said chamber would be capped by the pressure limiter at each vibration, so that the tongs would open.        